1. Field of the Invention
The invention relates to polycarbonate resins and their use in molding articles.
2. Brief Description of the Related Art
Aromatic polycarbonate resins are a well known class of synthetic polymeric resins, generally prepared by the reaction of a polyhydric phenol with a carbonate precursor; see for example U.S. Pat. No. 3,028,365. Although such resins have been found to be thermoplastically moldable under a broad range of molding conditions, only select polycarbonate resin compositions are useful for blow-molding. This is due to the unique requirements of a thermoplastic resin for blow-molding operations; see for example the requirements for the branched polycarbonate resins described in U.S. Pat. Nos. 4,286,083 and 4,621,132. The branched polycarbonate resins differ from most thermoplastic polymers used for molding in their melt rheology behavior. Most thermoplastic polymers exhibit non-Newtonian flow characteristics over essentially all melt processing conditions. However, in contrast to most thermoplastic polymers, certain branched polycarbonates prepared from dihydric phenols exhibit Newtonian flow at normal processing temperatures and shear rates below 300 reciprocal seconds.
Newtonian flow is defined as the type of flow occurring in a liquid system where the rate of shear is directly proportional to the shearing force.
Two other characteristics of molten thermoplastic polymers considered to be significant for molding operations are melt elasticity and melt strength. Melt elasticity is the recovery of the elastic energy stored within the melt from distortion or orientation of the molecules by shearing stresses. Melt strength may be simply described as the tenacity of a molten strand and indicates the ability of the melt to support a stress. Both of these characteristics are important in extrusion blow molding, particularly in fabrication by extrusion blow molding of relatively large articles. Non-Newtonian flow characteristics tend to impart melt elasticity and melt strength to polymers thus allowing their use in blow molding fabrication.
In the conventional blow-molding operation, a tube of the heat-softened polycarbonate resin may be extruded vertically into a mold. The extrudate is then pressed unto the mold surfaces with a pressurized gas flow (usually air or inert gas), shaping the heat-softened resin.
In practice, the desired physical characteristics of a blow-moldable polycarbonate resin can be achieved by either high molecular weight or branched polycarbonate. For example polycarbonate resins can be branched by reaction with tetraphenolic compounds; see for example the description in U.S. Pat. No. 4,474,999 (Mark et al.). Due to better performance the latter is preferred. Currently, a branched resin is synthesized. The proper melt strength and viscosity is obtained by controlling the molecular weight and the branching level. It would be highly advantageous if the same rheological behavior could be achieved by reacting a linear polycarbonate, during the compounding process in such a say that a polymer is obtained with same rheological properties as the currently available branched resin prepared by synthesis. This latter synthesis is time consuming and expensive. Any "off-specification" material produced is wasted. These disadvantages are not associated with the production of linear polycarbonate resins.
By the method of the present invention, we were able to produce a polycarbonate resin possessing a certain degree of branching and molecular weight, via reactive extrusion. This was achieved by melt extruding a linear polycarbonate resin with a specific branching agent and an appropriate catalyst system.